Surface acoustic wave (SAW) devices have been in extensive use as bandpass filters in communication equipments such as a cellular phone and the like. With the enhancement of the performance of cellular phone and the like, there is an increasing demand to enhance the performance of the filters using the surface acoustic wave devices.
In the surface acoustic wave device, however, there is a problem of the passband shifts due to temperature changes. In particular, lithium niobate and lithium tantalate frequently used have high electromechanical coupling coefficients, and thus provide an advantage in achieving broad-band filtering characteristics. However, lithium niobate and lithium tantalite are of inferior temperature stability.
For example, in lithium tantalate, the temperature coefficient of frequency change is −35 ppm/° C. and the frequency varies greatly in the temperature range of −30° C. to +85° C. Therefore, it is necessary to lower the temperature coefficient of the frequency change.
Patent Document 1 (Japanese Patent Publication No. 2001-53579A) describes a device fabricated by bonding a SAW propagation substrate and a supporting substrate together with an organic thin film layer. The propagation substrate is a 30-μm-thick lithium tantalate substrate, for example, and is bonded to a 300-μm-thick glass substrate with a 15-μm-thick organic adhesive.
Patent document 2 (Japanese Patent Publication No. 2006-42008A) describes a SAW device fabricated by laminating a lithium tantalate substrate (125 μm in thickness) and a quartz glass substrate (125 μm in thickness) together with an adhesive. Specifically, according to (0030), an adhesive layer is required because the direct bonding of the supporting substrate and the propagating substrate results in the peeling and cracks.
Patent document 3 (Japanese Patent Publication No. Hei 06-326553A), Patent Document 4 (Japanese Patent No. 3774782B) and Patent document 5 (U.S. Pat. No. 7,105,980B) also describe SAW devices fabricated by bonding a SAW propagation substrate and a supporting substrate together.
Further, according to Patent Document 6 (Japanese Patent Publication No. 2005-229455A), oxidized layers of a thickness of 0.1 to 40 μm are formed on both main faces of a silicon supporting substrate, and a piezoelectric substrate is then bonded to the supporting substrate to produce a SAW device. The oxidized layer of silicon is indispensable for reducing the warping of the thus obtained composite piezoelectric substrate 1.
According to the description of (0007) and (0013) of Patent Document 7 (Japanese Patent publication No. 2002-135076A), the surface roughness of the substrate is 10 μm and it is difficult to make the thickness of an adhesive layer constant.
According to (0018) of Patent Document 8 (Japanese Patent Publication No. H09-167936A), even when the rotation angle θ, cutting angle, of a propagation substrate is varied, it is shown substantially same temperature characteristics for the cases of, for example, 36° Y, 40° Y, 42° Y and 44° Y.
According to FIG. 6 of Patent document 9 (Japanese Patent publication No. H02-37815A, it is shown dependency of temperature characteristics on the electrode thickness. According to the description, the electrode thickness does not influence on the temperature dependency of frequency.
According to FIG. 4 of Patent document 10 (Japanese Patent Publication No. 2005-65160A), it is used quartz substrates having Euler angles of 0°, 127° and 90° to show the relationship between standardized electrode thickness H/λ and TCF (frequency temperature coefficient) at 25° C.
The thermal expansion coefficient and Young's modulus of lithium tantatate are described in (0021) of Patent document 11 (Japanese Patent Publication No. 2008-301066A).
Pages 43, 44, 36 and 37 of Non-Patent Document 1 “How-to; Strength of Materials: Basics” (authored by Hirofumi IDE and published by THE NIKKAN KOGYO SHIMBUN, LTD.) describe the relationship of thermal expansion coefficient and Young's Modulus of a bonding material.
Pages 989 and 991 of Non-Patent Document 2 “Science of Silicon” (edited by USC Semiconductor Basic Technique Research Group and published on Jun. 28, 1996) describes the thermal expansion coefficient and Young's Modulus of silicon.
Non-Patent document 3 “Handbook of Glass Optics” (published by Asakura shoten on Feb. 28, 1963, page 792) describes data of borosilicate glass.
Patent document 12 (Japanese Patent Application No. 2009-40947: Japanese Patent publication No. 2009-278610A) relates to the present patent application.